There is a growing demand to use body panels made of various aluminum alloys to minimize the weight of the vehicle body. However, because body panels made of aluminum alloy may not be able to provide an adequate vehicle body strength by themselves, it has been proposed to use steel panels for major frame components and selectively use body panels made of aluminum alloy only in parts such as a roof outer panel that do not require a high mechanical strength and have large surface areas.
Galvannealed steel sheet is also a preferred material for vehicle body outer panels because of its high corrosion resistance and its capability to wear a highly smooth paint coating, but is known to form a relatively poor bonding when spot welded to aluminum sheet.
As a technology for ensuring a high welding strength between sheets made of two different metallic materials such as aluminum and steel materials, it is known to interpose a clad metal sheet consisting of two layers of aluminum and steel materials that are firmly joined together by rolling or the like between the sheets made of different materials that are desired to be joined together as proposed in Japanese patent No. 3355790. This ensures a high spot welding strength, but has the disadvantage to require the clad metal sheet as an additional component that adds to the cost.
Another problem when joining sheet metal components is galvanic corrosion which is caused when electrolyte such as rain water seeps into a gap between members made of different metallic materials. Japanese patent laid open publication No. 2000-272541 proposes a method for preventing galvanic corrosion in a vehicle body. In this method, non-through type rivets are used for joining two panel members, and a sealant is applied to parts remote from the rivets. However, the use of rivets requires a special piece of equipment which is otherwise unnecessary, and this adds to the manufacturing cost.